Long-sought subatomic particle ‘seen’ at last

A subatomic particle that was first predicted to exist before the discovery of Pluto, 85 years ago, has finally been spotted on Earth. Known as Weyl fermions, they are like electrons. But unlike electrons, they have no mass. Physicists found them inside a material made of the elements tantalum and arsenic. These fermions dart around and through it in strange and exciting ways.

“It’s definitely a big deal,” says Leon Balents. He’s a condensed matter theorist at the University of California, Santa Barbara.

The material is called tantalum arsenide. The newfound particles’ behavior gives this material metal-like features. Called a “semimetal,” it shares features with materials such as graphene, which is a sheet of carbon that’s just one atom thick. Its novel structure gives graphene unusual superstrong characteristics that have excited researchers over the last decade or so. “There are a lot of reasons to be interested in these materials,” notes Balents, who was not involved with the new fermion discovery.

Some scientists think that like graphene, tantalum arsenide could change the future of electronics. It could let devices use a fast-moving electrical current that easily evades any bumps or valleys in its path. Physicists can also use tantalum arsenide to learn more about Weyl fermions. These particles are stuck inside the material. But some physicists suspect free-floating Weyl fermions might also exist.

Fermions make up a family of subatomic particles that includes electrons, neutrinos and more. So far, all known fermions obey rules laid out by English physicist Paul Dirac in 1928. But shortly after that time, scientists proposed two more types of fermions. One of them was named for German mathematician and physicist Hermann Weyl. What makes these special: Weyl fermions would have no mass.

Physicists had suggested that electrons interacting in certain materials would create ripples of energy. These ripples should act just like Weyl fermions would in space.

Earlier this year, with that in mind, Su-Yang Xu, Zahid Hasan and their colleagues proposed that tantalum arsenide could hold Weyl fermions. Xu and Hasan are condensed matter physicists working at Princeton University in New Jersey. These researchers fired a beam of high-energy X-rays to peer deep inside crystals of tantalum arsenide. Then they studied the energies and motion of its electrons. And they saw the signature of massless particles. They appeared to be Weyl fermions.

“I’m kind of amazed that someone was able to really see these things experimentally so quickly,” Balents says.

Tantalum arsenide is the first “Weyl semimetal” scientists have found. In some ways, such a semimetal is similar to “topological insulators.” Those are relatively newfound materials that don’t conduct electricity well on the inside, but let electrons run laps around their surface. Tantalum arsenide does not have the same kind of interior. But it does have high-speed electron superhighways on its surface.

The new twist with the Weyl semimetal, Xu says, is that its surface electrons don’t race around a closed track. Instead, they move from one side to the other. Then they disappear into the material and pop back out on the opposite surface.

Weyl semimetals also are similar to graphene, Balents says. Both materials let electrons zip around at extreme speeds and act like they have no mass. All these features make Weyl semimetals an exciting possibility for future electronics, Hasan says.

Now that physicists have found two of the three fermion types, Xu says, discovery of the still elusive third kind — the so-called Majorana fermions — may be just around the corner. In recent years physicists have seen hints of them. But they still lack proof. Says Xu, Weyl semimetals might be a good place to find not only Weyl fermions, but Majorana particles as well.